Disc brake rotors

ABSTRACT

A disc brake rotor ( 10; 50 ) comprising a mounting portion ( 12 ) and two friction portions ( 16, 18 ). One of the friction portions ( 16 ) is supported by the mounting portion ( 12 ) and the other by vanes ( 32 ) extending between the friction portions. Said vanes define cooling ducts ( 34 ) the mounting portion ( 12 ) also defines a plurality of inlets ( 42 ) through which air can pass to said ducts ( 34 ). Each inlet ( 42 ) is defined by a bounding surface ( 42   a ) which includes a section ( 44; 52 ) extending between the circumferential extremities ( 43; 53 ) of the inlet. Said section ( 42; 52 ) faces away from the friction portion ( 16 ) supported by the mounting portion ( 12 ), said section being continuously curved, symmetrical about an axial centre-line of the inlet, and extending axially less than half its circumferential extent.

FIELD OF THE INVENTION

This invention is concerned with disc brake rotors.

BACKGROUND ART

A disc brake rotor is arranged to rotate with a member, such as a wheelof a vehicle or a rotating part of a machine. Such a rotor provides twooppositely-facing annular friction surfaces which, in the operation ofthe brake, are engaged by blocks of friction material to decelerate therotor and hence the member. Two friction material blocks are moved(usually by hydraulic means) towards one another into contact with thetwo friction surfaces so that frictional forces occur slowing therotation of said rotor, and hence of said member. These frictionalforces generate considerable amounts of heat which has to be absorbed bythe rotor and causes its temperature to rise. If the rotor becomes toohot, the braking performance is adversely affected and the rotor wearsrapidly. Thus, such rotors need to have a significant thermal capacityin order to avoid rapid temperature rises.

In order to reduce temperature rises in disc brake rotors, it isconventional to form the rotor so that it comprises a first generallydisc-shaped friction portion which provides one of said friction annularsurfaces, and a second generally disc-shaped friction portion whichprovides the other of said annular friction surfaces. Said first andsecond portions are of constant thickness and are arranged in spacedparallel relationship. These portions are joined by vanes between whichare cooling ducts extending radially outwardly of the rotor. The coolingducts are arranged so that, as the rotor is rotated, air passes throughthe ducts and acts to cool said portions of the rotor on their oppositesides to said annular friction surfaces. Entrances to said ducts areprovided at an inner edge of said first and second portions and therotor functions as a centrifugal fan driving air outwardly to exits atthe outer edges of said portions.

A conventional disc brake rotor comprises a mounting portion whichextends axially between an end thereof which is adapted to be mounted onthe hub and an opposite end thereof which supports one of the frictionportions of the rotor. The other friction portion is supported by thevanes extending between the friction portions. Most rotors of this typehave said other friction portion supported further from the end of themounting portion which is mounted on the hub than the first mentionedfriction portion. This means that there is free access for air to theentrances to the ducts since the space between the two friction portionsis clear of the end of the mounting portion. These rotors arecategorised as being of the “inboard feed type”.

Rotors of the inboard feed type described above suffer from a problemknown as “coning”. This problem is caused by the heat generated bybraking causing the friction portions of the rotor to be hotter than themounting portion thereof. The higher temperature of the friction portionwhich is supported by the mounting portion causes greater thermalexpansion than that of the mounting portion so that, because the rotorportions are off-set from the mounting position on the hub, the junctionarea between the mounting portion and the friction portion is stressedradially outwardly and the friction portion tends to bend out of theradial plane. This effect is increased by the effect of the thermalexpansion of the other friction portion. Thus, the friction surfacescome out of the radial plane into a conical form which results in unevencontact with the brake blocks creating uneven heating and uneven wear.

The problem of coning can be reduced by designing the rotor to be of the“outboard feed type”. In this type of rotor the friction portion whichis not directly supported by the mounting portion is arranged to extendaround the mounting portion at a position nearer to the mountingposition on the hub. This means that instead of enhancing the bendingeffect of the friction portion which is directly mounted on the mountingportion, the expansion of the vane-supported friction portion acts inthe opposite direction and in practice overcomes the effect of the otherfriction portion, causing coning in the opposite sense but of lesserextent. Thus, the mounting portion is subject to compressive forces dueto the coning.

Although rotors of the outboard feed type have a reduced coning effectthey suffer from the disadvantage that the mounting portion obstructsthe entrances to the air ducts. The air has, therefore to enter througha gap between the inner periphery of the friction portion and the outerperiphery of the mounting portion. However, because air has to follow anintricate path to reach the gap, the cooling is compromised. Thisproblem has been addressed in one known rotor design by providingadditional inlets in the mounting portion so that additional air canenter the ducts. In this design, these additional inlets have been keptrelatively small in order to prevent the mounting portion from beingsignificantly reduced in strength, which is an important factor sincethis mounting portion must support high torques during braking. Theadditional inlets are slightly elongated in the circumferentialdirection so that they have parallel circumferentially extending sidesjoined at their ends by semi-circular sections. In this known design,the inlets occupy considerably less than half of the circumferentialextent of the mounting portion.

The present applicants experimented with a rotor design of the outboardfeed type with additional inlets of the type referred to above but ofincreased size. The objective was to increase the air flow to the ductsand also to reduce the stiffness of the mounting portion therebyenabling it to expand when the friction portion expands, therebyreducing the stress induced during thermal expansion. These experimentsrevealed that increasing the size of the additional inlets produced afurther undesirable effect. It was found that the rotor was subject tolarge variations of stress around the circumference of the junction areabetween the mounting portion of the rotor and the friction portionsupported thereby. This gave rise to a grave risk of cracking in thisarea.

The object of the present invention is to provide a rotor in which theproblem of coning is reduced, in which the air flow through the ducts isincreased, and in which the aforementioned large stress variations areavoided.

SUMMARY OF THE INVENTION AND ADVANTAGES

The invention provides a disc brake rotor arranged to rotate with a hubabout an axis and providing two oppositely-facing annularradially-extending friction surfaces which, in the operation of thebrake, are engaged by blocks of friction material to decelerate therotor and hence the hub, the rotor comprising a mounting portionextending axially between an end thereof which is adapted to be mountedon the hub and an opposite end thereof, the rotor also comprising twofriction portions each of which provides one of said annular surfacesthe friction portions being arranged in spaced parallel relationshipwith one of said friction portions being supported by said opposite endof the mounting portion and the other friction portion being positionedso that it extends around the mounting portion and is supported by vanesextending between the friction portions, said vanes also definingcooling ducts, the cooling ducts being arranged so that, as the rotor isrotated, air passes through the ducts and acts to cool the frictionportions, the mounting portion also defining a plurality of inletsthrough which air can pass to said ducts, the inlets being distributedcircumferentially around said mounting portion, characterised in thateach inlet is defined by a bounding surface which includes a sectionextending between the circumferential extremities of the inlet, saidsection facing away from the friction portion supported by the mountingportion, said section being continuously curved, symmetrical about anaxial centre-line of the inlet, and extending axially less than half itscircumferential extent.

In a disc brake rotor according to the invention, the special shape ofthe inlets enables the size of the inlets to be increased withoutjeopardising other aspects of the rotor's performance. Specifically, theshape of the inlets enables stress to be substantially equalised aroundthe circumference of the mounting portion. The use of larger inletsenables greater airflow to be achieved and also gives greater abilityfor the mounting portion to expand. This increased ability to expand isadvantageous since it reduces another form of distortion which is termed“buckling”. Buckling is a wave-like distortion extending around arotor's friction portion caused by the mounting portion resistingthermal expansion of the friction portion.

The shape of said section of the bounding surface of the inlet isdesigned to substantially equalise the stress around the circumferenceof the mounting portion, this stress being essentially in the axialdirection. The shape may be an arch-like shape, for example, the shapemay be that of half of an ellipse having its major axis alignedcircumferentially of the mounting portion.

The remainder of the bounding surface of the inlet is preferablydesigned to minimise stress also. Preferably, the said remainder issymmetrical about said axial centre-line. Said remainder may be formedby two elliptical sections joined by a section which extendscircumferentially or may be formed by an elliptical section such as ahalf ellipse.

Preferably, in order to increase the air flow, the transverse cross-sectional area of each duct decreases progressively between an entranceto the duct and an intermediate region thereof and increases betweensaid intermediate region and an exit of the duct, the surfaces of thefriction portions which bound the ducts extending as convex curvesbetween entrances of the ducts and exits thereof. The variation of saidtransverse cross-sectional area of the ducts may be achieved byvariation in the thickness of said friction portions of the rotor.

It is found that, in a rotor according to the invention, the totalextent of said inlets circumferentially may be more than half of thecircumferential extent of the mounting portion.

In order to reduce noise created during braking by reducing the possiblemodes of vibration, the number of inlets may be a prime number greaterthan or equal to seven. For the same reason, additionally oralternatively the number of vanes is a prime number which is differentfrom the number of inlets and is greater than eleven. It is alsodesirable if the number of studs by which the rotor is attached to thehub is a prime number. It is also desirable if none of the studs, inletsand vanes are aligned with one another in the radial direction.

BRIEF DESCRIPTION OF THE DRAWINGS

There now follow detailed descriptions, to be read with reference to theaccompanying drawings, of two disc brake rotors which are illustrativeof the invention.

FIG. 1 is a prospective view with parts broken away of the firstillustrative rotor:

FIG. 2 is a vertical cross-section taken through a portion of the firstillustrative rotor:

FIG. 3 is a view on a larger scale than FIG. 1 of an inlet of the firstillustrative rotor; and

FIGS. 4 and 5 are views similar to FIGS. 1 and 3, respectively, but ofthe second illustrative rotor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The first illustrative disc brake rotor 10 is made of cast iron and isarranged to rotate with a hub (not shown) of a vehicle about an axis 11.The rotor 10 provides two opposite facing annular radially extendingfriction surfaces 20 and 22. In the operation of the brake, the frictionsurfaces 20 and 22 are engaged by blocks of friction material (notshown) to decelerate the rotor 10 and hence the hub on which it ismounted.

The rotor 10 comprises a mounting portion 12 which extends axiallybetween an end thereof which is adapted to be mounted on the hub and onopposite end thereof. The first-mentioned end of the portion 12 isformed by an annular plate-like portion 12 a. Specifically, theplate-like portion 12 a has four holes 14 therein in which studs (notshown) on the hub are received in conventional manner. The mountingportion 12 also comprises a cylindrical portion 12 b which projects fromthe outer periphery of a portion 12 a and fits around said hub. Theportion 12 b extends to said opposite end of the mounting portion 12.

The rotor 10 also comprises two friction portions 16 and 18 whichprovide the two oppositely-facing annular friction surfaces 20 and 22.The friction portions 16 and 18 are arranged in spaced parallelrelationship with the friction portion 16 being supported by saidopposite end of the mounting portion 12. Specifically, the mountingportion 12 makes an annular junction with the friction portion 16. Theportions 12,16 and 18 of the rotor 10 are integrally cast out of greycast iron.

The friction portion 18 is positioned axially nearer to the plate-likeportion 12 a of the mounting portion 12 than the friction portion 16.The friction portion 18 extends around the portion 12 b and is supportedby vanes 32 which extend between the two friction portions 16 and 18,there being an annular gap 31 between the inner periphery of thefriction portion 18 and the portion 12 b of the mounting portion. Thevanes 32 are integrally cast with the portions 12,16 and 18.

The portion 16 is shaped generally as an annular plate bounded at itinner edge by its connection with the cylindrical portion 12 b, and atits outer edge by an axially-extending cylindrical surface 24. Theportion 16 is also bounded by the friction surface 20, which is planarand extends radially, and by a convex surface 26. Because of thecurvature of the surface 26, the portion 16 is at its thickest at aradially intermediate region thereof and is of lesser thickness adjacentto its inner and outer edges. The portion 18 is bounded at its outeredge by an axially-extending cylindrical surface 28 which has the sameradius as the surface 24. The portion 18 is also bounded by the frictionsurface 22 which is planar and extends radially, facing in the oppositedirection to the surface 20. The portion 18 is also bounded by a convexsurface 30 which is similar to the surface 26 which it faces exceptthat, at its radial inner edge, the surface 30 bounds an annular gap 31between the cylindrical portion 12 b and the friction portion 18.

The vanes 32 also define cooling ducts 34 which extend radiallyoutwardly and are arranged so that, as the rotor is rotated, air passesthrough the ducts 34 and acts to cool the friction portions 16 and 18.In FIG. 1, the vanes 32 which are visible have been sliced through in aplane normal to the axis about which the rotor 10 rotates so that onlythe junctions between the vanes 32 and the portion 16 are visible. Thevanes 32 serve to support the portion 18 (only part of the portion 18 isshown in FIG. 1). The vanes 32 project from the surfaces 26 and 30 atequal circumferential intervals, there being thirty-seven such vanes 32.The ducts have entrances 36 bounded by the inner edges of two adjacentvanes 32, and by the surfaces 26 and 30. The ducts have exits 38 betweenthe outer edges of the vanes 32. Between its entrances 36 and its exit38, each duct 34 is bounded by two adjacent vanes 32 and by portions ofthe surfaces 26 and 30.

At any point along its length, the transverse cross-sectional area of aduct 34 depends on the spacing of the adjacent vanes 32 and on thespacing of the surfaces 26 and 30. A controlled variation of thistransverse cross-sectional area of the duct 34 is achieved by thevariation in the thickness of said friction portions 16 and 18 of therotor caused by the convexity of the surfaces 26 and 30. Even though thevanes 32 get progressively further apart with increasing radius, theconvexity of the surfaces 26 and 30 is such that the transversecross-sectional area of each duct 34 decreases progressively between itsentrance 36 and an intermediate region 40 of the duct 34 where thesurfaces 26 and 30 have their closest approach. The intermediate region40 is substantially opposite the radial centre of the friction surfaces20 and 22. The transverse cross-sectional area of the duct 34 increasesbetween said intermediate region 40 and the exit 38 of the duct 34.

In the operation of the first illustrative rotor 10, rotation of therotor causes air to enter the gap 31 between the cylindrical portion 12b and the friction portion 18. The gap 31 therefore acts to allow air topass to the entrances 36 of the ducts 34. In order to improve the airflow through the ducts 34, and therefore the cooling of the rotor 10,the mounting portion 12 also defines seven inlets 42 through which aircan pass to said ducts 34. The inlets 42 are provided in the cylindricalportion 12 b in the area thereof which are aligned in the same radialplane as the friction portion 18 but extend axially opposite theentrances 36 of the ducts 34. These inlets 42 are in the form of holesthrough the portion 12 b and are equally distributed circumferentiallyaround the portion 12 b.

The inlets 42 have a special shape which is shown in FIG. 3. Each inlet42 is defined by a bounding surface 42 a. The bounding surface 42 aincludes a section 44 which extends between the two circumferentialextremites 43 of the inlet 42. The section 44 faces away from thefriction portion 16 which is supported by the mounting portion 12. Ascan be seen from FIG. 3, the section 44 is continuously curved, issymmetrical about an axial-centre line 47 of the inlet 42, and extendsaxially less than half its circumferential extent, ie the length of aline 45 joining the circumferential extremities 43 is more than twice aslong as the line 47 joining the section 44 to the line 45.

The remainder of the shape of each inlet 42 is defined by furthersections of the bounding surface 42 a. Specifically, these sections area straight section 48 which extends circumferentially of the rotor 10and two sections 46 which join the extremities 43 to the ends of thesection 48. These sections 46 are each in the form of a quarter of anellipse.

The air entering the gap 31 and the inlets 42 is accelerated bycentrifugal force along the ducts 34 until it reaches the intermediateregion 40 of the duct, where the transverse cross-sectional area of theduct reaches its minimum. This acceleration is caused by the decreasingtransverse cross-sectional area of the duct 34. At the intermediateregion 40, which is arranged to be directly opposite to the points atwhich the blocks of friction material engage the surfaces 20 and 22, theair reaches its maximum velocity, thereby increasing the coolingefficiency in this region. After passing through the intermediate region40, the air decelerates until it passes out through the exits 38 of theducts 34.

The second illustrative disc brake rotor 50 is shown in FIGS. 4 and 5.The rotor 50 is similar to the rotor 10 and like parts thereof are giventhe same reference numerals. The rotor 50 differs from the rotor 10 inthat the portion 12 b extends further axially than that of the rotor 10,in that the portion 12 a has eight holes 14, in that the vanes 32 arearranged differently, and in the shape of the inlets 42.

In the rotor 50, the vanes 32 are arranged in three concentric rows witha centre row off-set from the other two rows circumferentially. This isadvantageous as it reduces stress induced by one of the frictionportions 16 or 18 expanding further than the other.

The shape of the inlets 42 of the rotor 50 is illustrated in FIG. 5. Inthis case, the bounding surface 42 a has two sections 52 and 54. Thesection 52 faces away from the friction portion 16, is continuouslycurved, is symmetrical about the axial centre line 55 of the inlet,extends between the circumferential extremities 53 of the inlet, andextends axially less than half of the circumferential extent of theinlet. Specifically, the section 52 is in the shape of a half ellipsewith its major axis on the line 56 joining the extremities 53. Thesection 54 joins the section 52 at the extremities 53 and is in theshape of a half ellipse with its major axis on the line 55 and its minoraxis on the line 56.

1 A disc brake rotor arranged to rotate with a hub about an axis andproviding two oppositely-facing annular radially-extending frictionsurfaces which, in the operation of the brake, are engaged by blocks offriction material to decelerate the rotor and hence the hub, the rotorcomprising a mounting portion extending axially between an end thereofwhich is adapted to be mounted on the hub and an opposite end thereof,the rotor also comprising two friction portions each of which providesone of said annular surfaces the friction portions being arranged inspaced parallel relationship with one of said friction portions beingsupported by said opposite end of the mounting portion and the otherfriction portion being positioned so that it extends around the mountingportion and is supported by vanes extending between the frictionportions, said vanes also defining cooling ducts and entrances to saidducts, the cooling ducts being arranged so that, as the rotor isrotated, air passes through the ducts and acts to cool the frictionportions, the mounting portion also defining a plurality of inletsthrough which air can pass to said ducts, the inlets being distributedcircumferentially around said mounting portion, characterised in thateach inlet is defined by a bounding surface which includes a sectionextending between the circumferential extremities of the inlet, saidsection facing away from the friction portion supported by the mountingportion, said section being continuously curved, symmetrical about anaxial centre-line of the inlet, and extending axially less than half itscircumferential extent, the inlet extending axially opposite to theentrances of said cooling ducts between the friction portions. 2 A discbrake rotor according to claim 1 characterised in that said section ofthe bounding surface of the inlet has an arch-like shape. 3 A disc brakerotor according to either one of claims 1 or 2 characterised in thatsaid section of the bounding surface of the inlet has a shape which isthat of half of an ellipse having its major axis alignedcircumferentially of the mounting portion. 4 A disc brake rotoraccording to any one of claims 1 to 3, characterised in that theremainder of the bounding surface of the inlet is symmetrical about saidaxial centre-line, and is formed by two elliptical sections joined by asection which extends circumferentially. 5 A disc brake rotor accordingto any one of claims 1 to 3, characterised in that the remainder of thebounding surface of the inlet is symmetrical about said axialcentre-line, and is formed by an elliptical section. 6 A disc brakerotor according to any one of claims 1 to 5, characterised in that thetransverse cross-sectional area of each duct decreases progressivelybetween an entrance to the duct and an intermediate region thereof andincreases between said intermediate region and an exit of the duct, thesurfaces of the friction portions which bound the ducts extending asconvex curves between entrances of the ducts and exits thereof. 7 A discbrake rotor according to claim 6, characterised in that the variation ofsaid transverse cross-sectional area of the ducts is achieved byvariation in the thickness of said friction portions of the rotor. 8 Adisc brake rotor according to any one of claims 1 to 7, characterised inthat the total extent of said inlets circumferentially is more than halfof the circumferential extent of the mounting portion. 9 A disc brakerotor according to any one of claims 1 to 8, characterised in that thenumber of inlets is a prime number greater than or equal to seven. 10 Adisc brake rotor according to any one of claims 1 to 9, characterised inthat the number of vanes is a prime number which is different from thenumber of inlets and is greater than eleven. 11 A disc brake rotoraccording to any one of claims 1 to 10 in which the mounting portion isflared in a radially outward direction at said opposite end supportingsaid one friction portion and characterised in that said section of theboundary surface of each said inlet is radially closer to said ductentrances than the remainder of the boundary surface.